Tax treatment of put option calendar spread when each legs is closed in different tax years?

Your question is a can of worms.

Complex trades like spreads have offsetting positions raise complex tax treatment issues geared toward preventing taxpayers from tax (deducting losses and expenses from the losing side of a complex trade in the current tax year while deferring income on the offsetting winning position until a subsequent tax year).

Here's an article about this from GreenTraderTax which I consider to be a reputable tax advice site. Some excerpts are:

Offsetting Positions IRS Pub. 550: Capital Gains & Losses: Straddles defines an “offsetting position” as “a position that substantially reduces any risk of loss you may have from holding another position.”

“Loss Deferral Rules”in IRS Pub. 550 state “Generally, you can deduct a loss on the disposition of one or more positions only to the extent the loss is more than any unrecognized gain you have on offsetting positions. Unused losses are treated as sustained in the next tax year.”

Frankly, the offsetting position rules are complex, nuanced and inconsistently applied. There are insufficient tools and programs for complying with straddle loss deferral rules. Brokers don’t comply with taxpayer wash sale rules or straddle loss deferral rules on Form 1099Bs or profit and loss reports. Few local tax preparers and CPAs understand these rules, let alone know how to spot them on client trading records.

The IRS probably enforces wash sale and straddle loss deferral rules during audits of large taxpayers who are obviously avoiding taxes with offsetting positions. They make a lot of money, but it’s always deferred to the next tax year. The IRS doesn’t seem to be questioning wash sales and straddles during exams for the average Joe Trader.

I have traded calendar and diagonal spreads for decades. For legs closed or expiring in the current calendar year, I claimed the realized capital gain or loss and carried the other leg into the next year. I've never been flagged on it. Is this following IRS rules correctly or maybe not and the the IRS ignored me because I'm just an average Joe Trader? I don't know. So it boils down to Deductor Emptor (my variation of Caveat Emptor).